Get Sphinx release number from Git (if available)

The release number is not visible in the HTML version on readthedocs.org (if I'm not mistaken), it is, however, visible in the PDF version: http://readthedocs.org/projects/pysoundfile/downloads/pdf/gitrelease/ (this is a temporary link!).

If `git` is not available on the command line or if the documentation is built outside of a Git repository, the release number is set to `'<unknown>'`.

I disabled the `version` attribute, I don't know where this is used ...